Secondary emission enhancer for an x-ray image intensifier

ABSTRACT

The number of electrons emitted from a metal surface that is being bombarded by x-rays is increased by utilizing a secondary electron emission process within an insulator, potassium chloride (KCl), that is deposited on a metal, gold (Au) layer. The problem of obtaining a maximum electron yield from these layers is one of optimizing the secondary electron emission in the KCl for a given x-ray energy. This optimization is a function of the thickness of the KCl, its density, temperature, and an internal electric field.

United States Patent Jacobs et al.

Jan. 9, 1973 541 SECONDARY EMISSION ENHANCER 2,802,963 8/1957 Sheldon... .....313/65 R FQR AN X-RAY IMAGE INTENSIFIER 5,833,081 12;]958 Barbier .315/11 ,2 1,37 I1 1940 F th.. 3l3 [00X lnvenwrsi l Jacobs, Evansmn, 111-; 3,505,558 4/1970 ..31s/1i Mi ha Kovac, Princeton, 3,368,077 2/1968 Kazan ..250/213 VT [73] Ass1gnee: Elioflhwestern University, Evanston, Primary Examiner Anthony L- Birch Attorney-Louis Bernat [22] Filed: April 29, 1970 21 App1.No.: 32,972 [57] ABSTRACT The number of electrons emitted from a metal surface [52] U s C] 250/213 VT 313/68 313/95 that is being bombarded by x-rays is increased by d 315/12 utilizing a secondary electron emission process within [51] Int Cl ij 31/50 an insulator, potassium chloride (KCI), that is [58] Fie'ld VT 213 deposited on a metal, gold (Au) layer. The problem of 94 100 315/ obtaining a maximum electron yield from these layers is one of optimizing the secondary electron emission in the KCl for a given x-ray energy. This optimization [56] References cued is a function of the thickness of the KCI, its density, UNITED STATES PATENTS temperature, and an internal electric field.

3,235,737 2/1966 Niklas ..250/2l3 VT 11 Claims, 6 Drawing Figures X" RAYS M L SUBSTRATE 4' 0-0 0 O 0 Au 0 4 53 KC! MESH L j 56 COLLECTOR l'lHllll PATENTEUJAN 9|975 3.710.125

sum 1 0F 4 JOHN E" JACOBS MICHAEL a. xovac By /W% I PATENTEUJAH 9 I875 3.710.125

SHEET u or 4 MESH ELECTRODE .L suasTRATE fi 1 I T D O O O O 0 AU% 4 53 L w g MESH F 1 COLLECTOR "T- 6 60 66 65 67 If '2 62 N 7/ 6/ Way 6 '32; l NW x- RAYS 63 SCANNING GUN K ELECTRON WINDOW MULTIPLIER ELECTRODE GAS CHAMBER INVENTORS. MOSAIC TARGET MONIT0R JOHN E. JACOBS MICHAEL 6. K0 vAc 67 5 .W/I MLM/ o o o BY SECONDARY EMISSION ENHANCER FOR AN X- RAY IMAGE INTENSIFIER The invention described herein was made in the course of work under a grant or award from the Department of Health, Education, and Welfare.

In general, this invention relates to x-ray detection and more particularly to means for increasing the release of secondary electrons which occur in response to x-rays, in order to enhance the resulting image.

Reference is made to an article by the inventors in the Journal of Applied Physics, February 1970, entitled Enhanced X-Ray Photoelectric Yields From Metal- Insulator Layers. This article and a dissertation having a similar title, by the inventor Kovac, describe the invention in great detail. The dissertation (dated June 1970) is available at the Northwestern University, School of Electrical Engineering, in Evanston, Illinois.

The invention relates to a metal-insulator couple which releases high levels of secondary electrons when bombarded by x-rays or gamma rays. There are many uses for such a couple. However, a particular use resides in the bio-medical sensor field.

In general, x-rays are often used to study human organisms. The resulting x-ray dosage must be limited to a finite level which is not hazardous to health. As a result, the images tend to have a limited quality. The prior art has already explored the possibility of image improvement by making better use of the phosphors, and little room remains for further improvement in this area. If a better x-ray picture is to be obtained, it appears that such improvement must come about by electronic processing.

Electronic processing generally depends upon an intensification of an image produced by the x'rays. One of the more fruitful methods of intensifying an x-ray image comes about by releasing a cloud of free electrons which are a attracted to the image areas and thereby intensify the image. These free electrons may come from any of a number of different types of electron sources. Improved and more efficient electron sources provide a better means for greatly improving the intensification of x-ray images.

Accordingly, an object of the invention is to provide newer and more efficient sources of electrons. In this connection, an object is to provide improved secondary emission. More particularly, an object is to provide improved metal-insulator secondary electron emitters.

Another object of the invention is to improve x-ray images. Here, an object is to provide enhanced x-ray photoelectric emission from metal-insulator layers.

Still other objects of the invention are to provide more efficient secondary emission at lower cost and without requiring a substantial outlay of capital for entry into the market place.

In keeping with an aspect of the invention, these and other objects are accomplished by a metal-insulator layer using gold (Au) and an alkali halide, here potassium chloride (KCl). The maximum electron yield is obtained by optimizing the secondary electron emission in the KC] material. The optimization is a function of the thickness, density, temperature, and an internal electrical field.

The nature of exemplary equipment for accomplishing these and other objects and features of the invention may become more apparent from a study of the following drawings, in which:

FIG. 1 shows three schematic representations of layer and substrate configurations for studying current emitted from the metal and insulation layers due to xray absorption;

FIG. 2 graphically indicates the emitted current yields from various layers and combinations of layers, at room temperature, when subjected to a single pulse of x-rays;

FIG. 3 graphically shows time and temperature dependence of KC] layers evaporated on a gold layer of optimum thickness;

FIG. 4 is a perspective view of a three layer structure comprising a metal layer, an insulator layer, and a mesh electrode deposited on a substrate;

FIG. 5 is a schematic disclosure of the bias potentials applied to said couple and a collector electrode; and

FIG. 6 is an exemplary display system including a cathode ray tube using the inventive couple.

The invention is concerned with photoelectric currents caused by the transmission of x-rays through a couple comprising metal-insulator layers. In one exemplary construction, the metal portion of this couple is a layer of gold and the insulation is an alkali-metal chloride, potassium chloride (KCl) deposited on a conductive substrate.

FIG. 1 shows how each of these layers is connected into an electrical circuit, and an emitted current I, is measured when the couple is subjected to x-ray bombardment. FIG. 2 shows the characteristic curves which are found when the emitted currents I, are measured under the layer construction conditions depicted in FIG. 1. In order to make these measurements, gold is used as the metal sinceit has a high absorption in the energy range of 0-80 KV peak values. The potassium chloride insulater material (KCI) is used since it has a high secondary emission coefficient. When drawing FIG. 2, the values of emitted current were normalized so that unity on the vertical scale corresponds to the current value flowing when the layer thickness reached the gold plateau current. The values on the vertical scale indicate secondary emission ratio, plotted on a scale having a base line corresponding to the maximum current that can be obtained from a gold layer alone.

The layers are vacuum deposited on glass substrates. To make this deposited couple, an initial layer (1,000 A to 1,500 A) of aluminum is first deposited on the glass to serve as an electrical backing layer. Then gold, which is the next to be vacuum deposited on the substrate, provides an emitted current, 1 when subjected to the radiation of a full wave rectified x -ray tube, rated KV peak and operated at 5 ma. The current so emitted from thegold layer (curve 3) increases as a direct function of the increasing thickness of the gold layer. A plateau in the emitted current is reached at about 2,000 A (Angstroms), as shown in FIG. 2. At this plateau, the thickness of the gold layer is indicative of the range of the most energetic electrons which are photoelectrically liberated from the gold film. Also, at the plateau, the effective x-ray energy is approximately 30 KV. If there ia an L shell interaction, the photoelectric absorption results in an electron with an initial kinetic energy of approximately 15 KV.

The insulator layer of an alkali halide layer, such as potassium chloride, (KCl), is evaporated over the gold layer in a vacuum of approximately 5 X 10 Torr. If curves are plotted to indicate the emitted current versus the layer thickness, it will be found that the slope of the KC] curve, curve 4, is substantially smaller than the slope of the gold curve, curve 3. This slope difference is due to the much smaller x-ray absorption coefficient for the KCl as compared with the coefficient of gold.

The sharp peak in curve 2 at a KCl layer thickness of approximately 1,000 A is indicative of the range (in thickness of the KCl) of the fastest electrons ejected from the gold film. After this peak, the emitted current, 1,, in the KCl layer slowly decreases. This decrease indicates a higher absorption of the electrons from the gold base. The curve 2 for KCl (bulk density) on gold asymptotically approaches the curve 4, obtained for the KCl alone. The peak current, I, for the Au-KCl configuration is approximately three times the current, I for the gold layer alone.

FIG. 2, curve 1, shows that the low density layers of KCl on gold give substantially higher yields than the bulk density layers on gold. In these curves, the horizontal scale values are for an equivalent thickness, which means the thickness that the KCl film would have if it were bulk density instead of low density.

FIG. 3 shows the temperature and time dependence of the current yield for the low density films. The results for bulk density are also included for comparison. For drafting clarity and readability, the zero point of time has been shifted slightly for each curve.

The secondary electron emission phenomenon in metals is known to be virtually independent of time and temperature. Therefore, the time and temperature effects shown in FIG. 3 result from processes within the KC] layer. The electron-phonon interaction causes the temperature effect upon the secondary electron emission from insulators. Assuming that the frequency of the optical longitudinal vibrations of KCl is 4.0 X sec there is an increase in secondary electron yield of approximately 21 percent when the temperature is reduced from 297 K to 180 K. Qualitatively this increase is demonstrated by comparing (FIG. 3) the initial emitted current I for the two temperatures.

However, it is not entirely clear why there is a greatly enhanced, time dependent yield that follows the time t=0, particularly in the low density film at low temperatures. In the low temperature, low density specimens the actual shape of the initial rise to saturation varies from sample to sample, but is reproducible for a given sample. Part of the enhanced yield is due to a surface charge building up on the emitting side of the porous KCl, thereby producing a field enhanced condition for the escape of electrons. This charge is known to monotonically increase with time, and then it reaches a saturation condition.

A photoelectric process results in the emission of a photoelectron from an atom when it is bombarded by an incident photon. The process consists of ejecting an orbital electron (usually from the K or L shell) from an atom of the absorbing material. The absorption cross section varies approximately as ZA where Z is the atomic number of the absorber and A is the wavelength of the incident radiation. Gold (Z=79) is used because of its high atomic number and the ease with which it can be handled. The effective KV of the x-ray photons has approximately 30 KV. Since the electrons in the K shell are bound by 80 KV, they do not contribute to the photoelectric absorption for the x-ray energy used.

After an electron has been ejected, an atom is left in an excited state. De-excitation can occur in several ways: First, after a photoelectron has been ejected, its place can be taken by an electron from a more distant shell with the simultaneous emission of characteristic radiation. The characteristic radiation is then available for further absorption. Second, the excited atom can undergo de-excitation by a non-radiative process. The atom reduces its state of excitation by cropping an electron from a more distant shell into the empty state left by the photoelectric ejection process, and simultaneously another electron is ejected from the atom.

Therefore, if a K electron has been photoelectrically ejected, there might be a transition wherein one of the L electrons drops to the K shell, and another L electron is emitted. The kinetic energy of the-ejected electron is the difference in energy between the excited K state and the excited state corresponding to the absence of two L electrons. An atom may emit two or more electrons in a sequence of such transitions.

The angle at which the photoelectron is ejected, relative to the angle of the incident x-ray, is important for consideration of spatial resolution. At low energies, the photoelectrons tend to be ejected in the direction of the electric vector of the incident radiation; hence, ejection is at right angles to the direction of incidence. At higher energies, the angular distribution is more in the forward direction. For a structure consisting of a 600-700 A layer of KCl on a 15-20 A layer of gold, the maximum gain is 8.4 when there is a primary voltage of 3.2 KV. The effect of the gold is to scatter the incident primaries so that they enter the KC] at angles other than normal. This gives the primaries a greater effective path length in the KCl, thus increasing the yield of secondary electrons, but the KCl has a limited life under electron bombardment. An investigation of the transmission characteristics of low density KCI instead of bulk density shows that the secondary. emission yields are almost an order of magnitude higher for the low density as compared to the yield for the bulk density.

Two time constants are necessary to describe the performance of the dynodes. The first constant is the time required for the KCl layer to charge up after a primary beam has been turned on. The surface charge creates an electric field across the layer of KCl which enhances the escape probability of the low energy secondary electrons. Fields as high as 5 X 10 V/cm can be built up in the film. The time required to charge the surface is inversely proportional to the primary beam current density and usually takes place within several seconds. The second time constant associated with the emission process is in response of a pre-charged dynode to a change in primary current. This. time constant is less than 5.5 X 10' sec. This time response is similar to the time response of a true secondary electron emission process.

Optical absorption bands can be introduced into the alkali halides (such as potassium chloride) by x-ray, -yray, neutron, and electron bombardment, introducing excess metal into the halide crystal, adding chemical impurities, and electron injection (electrolytic coloration). If a photon in the far ultraviolet (or any other form of energy greater than the band gap) is absorbed,

an electron is injected into the conduction band, thereby producing a photoconductivity. Absorption of light on the long wavelength side does not give rise to photoconductivity. There is, however, a strong optical absorption at energies that are somewhat less than the band gap energy. This absorption is due to the creation of an electron-hole pair bound to each other by coulombic attraction.

The mobile particle, consisting of an electron bound to a positive hole, is called an exciton. An exciton has the ability to move through a crystal, transporting energy, but not transporting any net electrical charge. The optical lifetime of an exciton is of the order of seconds. It is highly mobile during this lifetime and may undergo a displacement of 10- cm relative to the point of creation.

Excitons can play a significant role in the creation and ionization of F-centers. Basically, an F-center is an electron trapped at a negative ion vacancy at a level that is approximately 2.0 eV below the conduction band minimum. An exciton may transfer its energy to an F-center, thereby raising the trapped electron to the conduction band. Alternatively, an exciton may undergo de-excitation in the region of a negative ion vacancy resulting in the electron being trapped in the vacancy site. The F-centers produced by x-rays arise from the trapping of slow electrons at negative ion vacancies that are either produced by the x-rays themselves or were originally present in the crystal in the form of clusters. The x-rays disassociate the clusters and thus make the vacancies available for trapping electrons.

During the vacuum deposition, preferably, a small piece of potassium is placed in the bottom of a cylinder. After an initial evacuation of the cylinder to approximately l X 10 Torr., the system may be backfilled with argon and repumped. When the pressure is 5 Torr., the system is sealed off, and the cylinder is inserted into a hot oven. The potassium begins to boil, and its vapor rises, condenses on the cool upper section of the cylinder, flows down the inner wall, and then enters the hot zone where it is vaporized again. The action is similar to a diffusion pump. At equilibrium, the number of F-centers produced in the KCl is determined by the vapor pressure of the potassium. The backfilling argon is in equilibrium with the potassium vapor.

In addition to the foregoing phenomena, the major portion of the enhanced yield of emitted current in low density films at low temperature results from the build up and subsequent ionization of F-centers in the KCl insulation material. The x-rays produce F-centers according to a time-temperature dependent process. For a constant x-ray intensity at room temperature, the F- center concentration in the KC] material increases with time, and then it saturates. At lower temperatures, the F-center concentration increases more rapidly with time, and the saturation disappears. When measured under the same conditions, the photoconductive current saturates at all temperatures. There is an enhanced optical photoelectric yield due to exciton assisted emission from the F-centers.

In summary, it appears that the enhanced yield may be due to a number of interrelated complex mechanisms. Initially the x-rays produce F-centers in the KC] by breaking up the negative ion vacancy clusters and by directly creating negative ion vacancies. Then, these vacancies trap electrons to form F-centers. The F-center production is more efficient at low temperatures than at room temperature. As the x-ray irradiation continues, the surface charge builds up and causes the electric field across the layer to increase to values as high as 10 V/cm. At a certain critical value of electrical field, the F-centers are quickly ionized, causing the large peak in emission. Saturation is achieved when the surface charge has saturated, and the rate of emission from the F-center is equal to the rate of trapping.

With the foregoing principles in mind, it is thought that the following description of one exemplary structure will teach those who are skill-ed in the art about means for and methods of producing enhanced electron yields. For this, reference may be made to FIG. 4.

The substrate 50 is a glass (such as Pyrex-Coming 7740, and Vycor-Corning 7900) disc. In one case, this disc is 2 inches in diameter and ,4; inch 1/16 inch thick. The effective emitting surface is l inches in diameter. This large area permits emitted currents to be of a magnitude which is easily measured (such as 10 to 10' amperes). First, a 1,000 A layer 51 of aluminum is evaporated on the substrate to insure good adhesion to the substrate and prevent spring loaded contacts from scraping off the connections when the unit is placed in use. The aluminum layer also provides a uniform electric field under the gold. Next, a layer of gold 52 is evaporated over the aluminum. Thereafter, layers 53 of bulk density KCl are evaporated over the gold. Reagent grade (99.5 percent) KC] in powdered form is evaporated from an alumina crucible. The evaporated layer becomes foggy since KC] is very hygroscopic, and it absorbs water vapor. This absorption is minimized by using argon or dry nitrogen to backfill the vacuum system during the deposition.

The increased secondary electron yield in the thin insulating layer is due to the build up of an internal field. Therefore, that electric field is increased to realize even higher yields. To accomplish this, a fine metal mesh 54 of gold and aluminum is evaporated into the KC] layer, as shown schematically in FIG. 4. The mesh is made in two steps. First, horizontal strips are evaporated through a number of closely spaced parallel wires, and then vertical strips are evaporated by turning the wire mask by The KC] films should be free of pinholes and surface irregularities to prevent a short circuit between the evaporated mesh 53 and the gold base layer 52.

The bulk density KCl layer on a gold base produces an effective secondary electron gain of approximately 3. Substantially greater electron yields are obtained by using low density films of KCl. The thickness of these evaporated layers is monitored by using a quartz crystal oscillator which responds to the total mass deposited on it. Since the density of the deposited material is known, the thickness is easily calculated.

FIG. 5 shows how the electrons may be collected at 56 when x-rays energize the gold-insulator couple 52, 53. The gold layer is biased at a relatively high negative potential. The mesh 54 is biased positive relative to the potential of the gold layer.

FIG. 6 shows an exemplary structure using the inventive secondary electron source to enhance an x-ray image. Here, a glass envelope 60 has a scanning gun 61 at one end and the inventive gold-insulation couple 62 at the other end. An electron multiplier 63 collects electrons released as the gun 61 scans the back of the couple. A TV monitor is connected to the multiplier 63 to display an image of the electrons appearing on the couple 62 at the time of scanning.

The imaging tube 60 consists of two sections. The imaging section 65 and the reading section 66. The spacing between the electrodes is adjustable. The gas chamber is supplied with argon gas at slightly higher than atmospheric pressure. This excludes the atmospheric gases that are not as efficient as the Argon.

In operation, the x-ray photons impinge on the window electrode and liberate photoelectrons which are ejected into the gas. The released electron ionizes the argon gas along its path. Wider spacing between the electrodes gives more gain in the gas, but causes loss in resolution since some electrons travel through the chamber at angles other than normal. A spacing of mm gives good results. An adjustable voltage is applied to the window electrode (0 to 3,000 V D.C.).

The source of primary electrons is a standard photocathode 67 which gives up photoelectrons which are accelerated by approximately 7 KV and are focused into the target. These photoelectrons constitute a writing beam which is focused on the couple 62. The energy of the primary electrons is sufficient to penetrate the gold. The primary electrons dissipate most of their energy in the low density KCl by creating many low energy electrons.

The read side of the target 62 is the KC] side. The

exit surface potential of the KC! is established by the low energy scanning electron gun. This surface potential is approximately equal to the gun cathode potential since low energy scanning is used. The aluminum backplate is maintained at a positive potential (usually 10 to volts positive) with respect to the gun cathode. Therefore, the slow secondary electrons which are liberated by the absorption of the writing beam are in an electric field which causes them to travel to the backplate.

- The collection process is rapid since the electrons travel in the interstitial voids of the low density layers rather than in the conduction band. As the secondary electrons are collected by the signal plate, the exit surface potential (scanning gun side) moves toward the backplate potential. This is, in effect, a secondary electron conduction process.

As the scanning beam moves across the target, there is the variation in surface potential. The gun drives each individual element back to equilibrium potential. The replenishing of the charge on the surface is capacitively coupled to the backplate. This, then, is the video signal. The very high effective resistivity (10 ohmscm) of the KC] target makes long storage times possible. Since there is no dark current, long integration periods are possible.

Thus, an image 67 of a target 68 is displayed on the monitor. The drawing shows target 68 as a simple geometrical form. However, it should be understood that any suitable target may be used. The inventors primary interest is bio-medical usage. Non-destructive testing is another example of usage. Those who are skilled in the art will readily perceive still other uses for intensified secondary electron emission.

In operation, the x-radiation impinges on the window-electron emitting electrode 67. This electrode is either a thin aluminum foil or a wafer of glass on which silver or gold has been deposited. Photoelectrons are liberated by the absorbed x-radiation in the thin metal film and by absorption of the x-radiation or the x-ray induced photoelectrons in the gas.

The gas in chamber 69 is argon to which a small amount of methyl alcohol has been added, to act as quenching agent to assure image formation. The electric field applied across the chamber accelerates the photoelectrons which are then amplified by gas ionization. The resulting avalanche does not spread laterally through the chamber but remains confined to the area where the initial photoelectron was formed.

The gas discharge terminates at the opposite electrode. This electrode is a special mosaic consisting of a disk of insulating material into which a matrix of thin Conductive rods has been implanted. These rods provide discrete conductive paths from the gas chamber to the vacuum chamber and serve as a charge storage element. The elements are charged by the gas ions and discharged by the scanning beam 71.

In the vacuum chamber, a low velocity scanning gun 61 reads the charge pattern deposited on the array of rods. On the gas chamber side of the mosaic target a fine mesh has been etched. This metallic mesh is interlaced between the rods and is insulated from them. A voltage between the mesh and the window electrode provides the accelerating field for the avalanche.

The imaging-device is capable of detecting and displaying x-ray images with incident radiation levels of 20 milliroetgens picture frame.

Due to the combination of the gas amplification and charge storage capability of this device, it is anticipated that further technical development will result in a system capable of displaying the absorption of individual quanta.

The foregoing is a description of a preferred embodiment of the invention. However, it should be understood that the invention is not limited thereto. Quite the contrary, the invention includes all equivalent materials and structures. For example, the insulator material may be any suitable alkali metal halides, such as: lithium chloride, sodium chloride, potassium chloride (the preferred material), rubidium chloride, and cesium chloride. In addition, calcium tungstate or sodium tungstate may be used to a lesser degree of efficiency. Those who are skilled in the art will readily know how to use these materials in the inventive manner.

Therefore, the appended claims are to be construed to cover all equivalents falling within the true spirit of the invention.

We claim:

1. A couple for enhancing the emission of electrons which are yielded responsive to a bombardment of said couple by x-rays, said couple comprising a metal layer for high speed emission of electrons responsive to an absorption of photons by said metal, a layer of insulation in intimate contact with said metal layer, said insulation material being a material which emits secondary electrons responsive to the emission of said high speed electrons, and means for establishing a high gradient field across said insulation material.

2. The couple of claim 1 wherein said metal is a noble metal.

3. The couple of claim 1 wherein said metal is gold.

4. The couple of claim 3 wherein said insulator is potassium chloride.

5. The couple of claim 4 wherein said couple is deposited on a substrate having at least a conductive surface under said metal layer.

6. The couple of claim 5 wherein said means for establishing said high gradient field is a mesh electrode in contact with the insulator side of said couple.

7. The couple of claim 5 and a cathode ray tube, said couple being mounted in the end of said tube at a position where said insulation layer is scanned by said cathode ray, a television monitor, and means responsive to said scanning of said insulation layer by said cathode ray for supplying an image signal to said television monitor.

8. The couple of claim 7 and an x-ray source means for bombarding said couple with x-rays, whereby said insulation layer gives off secondary electrons in the image of a target between the source of said x-rays and said couple.

9. The couple of claim 8 and means comprising a rare gas amplifier interposed between the target and couple, and means responsive to bombardment of xrays upon said gas amplifier for releasing electrons from said metal layer.

10. A couple comprising a layer of gold and a layer of potassium chloride insulation material deposited on a substrate and positioned in an x-ray field, said potassium chloride having a density and a temperature for yielding high levels of secondary emission for producing F-centers.

11. The couple of claim 1 where said insulator is an alkali metal halide. 

2. The couple of claim 1 wherein said metal is a noble metal.
 3. The couple of claim 1 wherein said metal is gold.
 4. The couple of claim 3 wherein said insulator is potassium chloride.
 5. The couple of claim 4 wherein said couple is deposited on a substrate having at least a conductive surface under said metal layer.
 6. The couple of claim 5 wherein said means for establishing said high gradient field is a mesh electrode in contact with the insulator side of said couple.
 7. The couple of claim 5 and a cathode ray tube, said couple being mounted in the end of said tube at a position where said insulation layer is scanned by said cathode ray, a television monitor, and means responsive to said scanning of said insulation layer by said cathode ray for supplying an image signal to said television monitor.
 8. The couple of claim 7 and an x-ray source means for bombarding said couple with x-rays, whereby said insulation layer gives off secondary electrons in the image of a target between the source of said x-rays and said couple.
 9. The couple of claim 8 and means comprising a rare gas amplifier interposed between the target and couple, and means responsive to bombardment of x-rays upon said gas amplifier for releasing electrons from said metal layer.
 10. A couple comprising a layer of gold and a layer of potassium chloride insulation material deposited on a substrate and positioned in an x-ray field, said potassium chloride having a density and a temperature for yielding high levels of secondary emission for producing F-centers.
 11. The couple of claim 1 where said insulator is an alkali metal halide. 